OP TIDES. 



force, and the acceleration of the lunar tide produced by this cause 

 timst be greater than that of the solar ; hence it may happen that 

 when tire lunar tide occurs two or three hours after the transit of 

 the moon, the solar tide may be three or four hours after that of 

 the sun, so as to be about an hour later, at the times of conjunction 

 and opposition, and the tides will be highest when the moon passes 

 the meridian about an hour after the sun; while at the precise time 

 of the new and full moon, the lunar tide will be retarded about a 

 quarter of an hour by the effect of the solar tide. 



The particular forms of the channels, through which the tides an. 

 live at different places, produce in them a, great variety of local 

 modifications; of which the most usual is, that from the conver- 

 gence of the shores of the channels, the tides rise to a much greater 

 height than in the open sea. Thus at Brest the height of the tides 

 is about 2O feet, at Bristol 30, at Chepstovv 40, at St. Maloes 50; 

 and at Annapolis Royal, in the Bay of Fundy, as much sometimes as 

 100 feet ; although perhaps in some of these cases a partial oscilla- 

 tion of a limited portion of the sea may be an immediate effect of 

 the attraction of the luminary. In the Mediterranean the tides are 

 generally inconsiderable, but they are still perceptible ; at Naples 

 they sometimes amount to a foot, at Venice to more than two feet, 

 and in the Euripus, fora certain number of days in each lunation, 

 they are very distinctly observable, from the currents which they oc- 

 casion. In the West Indies, also, and in the gulf of Mexico, the 

 tides are less marked than in the neighbouring seas, perhaps on 

 account of some combinations derived from the variations of the 

 depth of the ocean, and from the different channels by whkh they 

 are propagated. 



In order to understand the more readily the effects of such com. 

 foinations, we may imagine a canal, as large as the river of the Ama- 

 zons, to communicate at both its extremities with the ocean, as to 

 receive at each an equal series of tides, passing towards the opposite 

 extremity. If we suppose the tides to enter at the same instant at 

 both ends, they will meet in the middle, and continue their progress 

 without interruption : precisely in the middle the times of high and 

 Jow water belonging to each series will always coincide, and the 

 effects will be doubled ; and the same will happen at the points, 

 where a tide arrives from one extremity at the same instant that an 

 earlier or a later tide comes from the other ; but at the intermediate 



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